Mucking around with Intersection Observer API

Published Oct 3, 2022
Updated Oct 23, 2022
By Simon

After working with scroll event listener & getBoundingBox to create a slide-in/up effect, as well as topPostion to make a sticky navigation and needing to calculate positions and recalculate positions I decided to see if there was a better way to trigger transitions and animations in JavaScript. The main reason I decided to investigate was that I could just picture for complex or multiple animations my code would become complex very quickly.

Related: How do I make an element slide in with 4 lines of JavaScript?
Related: How to make the Top Navigation background colour and size magically change on scroll

The Intersection Observer API

After researching, the Intersection Observer API came up as a solution. Sorry, I don't really know where and when I was pointed towards this, as I have spent a lot of time in the last few years mucking around with code and on MDN reading, but I am glad I took the time to look at it. On MDN it explains the Intersection Observer API was created to solve issues of browser performance that other methods have of working out the intersection of elements. In their words, using other methods to build up the necessary information becomes "unreliable and prone to causing the browser and the sites the user is accessing to become sluggish" and "since all this code runs on the main thread, even one of these can cause performance problems". Boom! exactly what I was thinking!

So the Intersection Observer API is something worth becoming familiar with if you want to create fast and modern sites with animations or transitions triggered by scroll. In this article, I will take a look at the Intersection Observer API and try to illustrate how it actually works with some practical examples.

Benefits of using Intersection Observer API

If you want to trigger actions based on scroll the Intersection Observer API is what you need as the performance is better than using other methods as it hands off all the work to the browser instead of needing the browser to parse, compile and run multiple lines of codes in the main thread. That's it, the Intersection Observer API gives greater performance.

How to set up Intersection Observer API

In this part, we will look at the set-up in more detail. I have broken it down into the steps below in a way that makes the most sense to me, I hope it also helps you.

Before we start we will need some HTML. We can use the same code from Github for the How to move an element in JavaScript article. Please follow the instructions from here on how to do that if you are unsure. If you use this repo remove all the JavaScript except the first line; const three = document.getElementById('three');

Okay, now we have a nice set up let's start writing some JavaScript code!

  1. In the JavaScript file, we first create an intersection observer by using the IntersectionObserver constructor, we do this by using the new keyword and assigning the new intersectionObserver  to the observer variable we created. See below for the code we have thus far.

     let observer = new IntersectionObserver( handleIntersect , options );

    The IntersectionObserver takes a callback function and options as you can see. However, before the callback can be called we need to set a target and also the options.

  2. The target is set on the observer using the observe method and we pass the target to it.

    You can think that we "observe the target".


    To set the target we can select any element using a class or id using querySelector or getElementByID. In the case below we are selecting an element with an ID of three. This code should already be present if you cloned the repository and followed the instructions above.

    const three = document.getElementById('three');

    So now this is our code.


    Now we have a target to observe from our new intersection observer all we need to do is add the options.

  3. There are 3 options we can set.

    root: The element that is used as the "boundary-line" for which the target is tracked to cross over. The viewport is implied if nothing is set, root: null. It has to be an ancestor of the target.

    rootMargin: this adds some offset to the root. It can have similar values to CSS margin, that is top, right, bottom, left and can be a percentage. In this example, I have a negative margin set because I want it inside the root, up from the bottom.
    It defaults to 0

    threshold: The threshold is set on the target. It is used to invoke the callback when the target crosses into the root intersection rectangle. This can be a single number or an array. It can be per cent or pixels value or values.
    It defaults to 0.

    Make an object like below and add it before creating the new intersectionObserver()

    let options = {
        root: null, // null mean uses viewport
        rootMargin: "0px 0px -250px 0px",
        threshold: [0.5 , 0.75],


So this is our code so far.

const three = document.getElementById('three');

let observer;

let options = {
  root: null, // null mean uses viewport
  rootMargin: "0px 0px -250px 0px",
  threshold: [0.5 , 0.75],

observer = new IntersectionObserver( handleIntersect , options );

Now that we have the options set the Intersection Observer API is ready to be used. However, as such nothing will happen as we need to create the callback function. The callback function will perform some action and will be custom JavaScript code so the sky is the limit.

Let's take a look at what happens when the Intersection Observer API is run. When the target crosses into the root element.

The Inner workings of the API

Once the Intersection Observer API is set up it is listening for the threshold we set to be met. Once the threshold is met, the callback is invoked and at that time the callback receives a list of the IntersectionObserverEntry interface properties and the observer. See the diagram below.

intersection observer API showing when a threshold is met and how the callback receives the entries

In most examples, I have seen these are passed back in as entries but you could use any name, in fact on W3C they are passed in as changes, but since the interface is named IntersectionObserverEntry let's use entry.

To see the values of the entries as you can see in the above illustration you can console.log them using console.log(entries); inside your callback function.

Okay, so we have a lot of properties with values we can use in our callback. So let's do something simple using the isIntersecting property.

function handleIntersect(entries) {
  entries.forEach((entry) => {

     if (entry.isIntersecting) {"up");
     } else {'up');


Having Multiple Thresholds set

Remember you can use an array of values for the threshold. How exactly does this work? If you use an array for the threshold option every time a threshold value is hit, the list of IntersectionObserverEntry interface properties is passed into the callback function. In this case, you would need to use a property other than isIntersection. The most likely property you would use is the intersectionRatio property. I will be looking at a more complex example in the weeks to come.

That's about it, under the hood this is what happens when you create a new intersection observer. Once you grasp this the sky is the limit and only your imagination and creativity and knowledge of using JavaScript will stand in your way.

On MDN there is an interesting demonstration where the threshold is an array and it changes the hue. When I first read that it seems quite confusing to tell the truth and thus I had to write it out in my own way. I suggest you look at those examples, however, as they are quite interesting. In a following article, I am going to add another example that works on adjusting the transform of an element on scroll using an array of 5 threshold values and also play around with colour a bit more so be sure to sign up for the newsletter and be the first to know.

Thanks for reading and look after yourself!  Carpe Diem!